1. Field of the Invention
This invention relates to an improvement in a double-ended mechanical writing implement in which backward and forward movements are controlled by a carrier having a spiral guide traveling along the spiral groove provided on the inside of a pencil barrel in response to the mechanical rotations of the cap holding one end of a lead supporting member. More particularly, this invention relates to an improvement in the structure of the cap and the carrier mentioned above.
2. Description of the Prior Art
A writing implement of this type was disclosed in U.S. Pat. No. 4,136,980 granted to the present inventor. In this implement, since a carrier of the pencil lead does not have an absolutely circular form, but a part annular form, a stable support of the lead could not be assured. Furthermore, the implement has a drawback in that a spiral guide defined on the outside of the carrier is naturally decreased in its whole length and, thus, the spiral guide cannot precisely be engaged with a spiral groove formed on the inside of the pencil barrel when the inner diameter of the barrel is either larger or shorter than that of a standard position. This is likely to cause the spiral guide to be removed from the spiral grooves.
Since the pencil barrel of the writing implement of such a type should be molded of a suitable synthetic resin by either an extruding or injection process, in the case of extrusion molding, there would be produced an error in the inner diameter of the barrel extruded ranging from 0.05 to 0.15 mm, while in the case of injection molding, such an error would not occur.
However, when the pencil barrel is extruded, the inside of the barrel must be tapered in order to facilitate the release of the outward and inward molds after completing the molding procedures. Therefore, it has been desired to provide such a writing implement with a carrier having a structure suitable to be precisely engaged with the spiral groove on the inside of the barrel. In addition, since the prior art writing implement has a neutral border member in the middle part of the inside of the barrel in order to prevent the carrier from a back movement, the carrier is apt to be excessively pushed backward when it is subjected to a back movement.